1. Field of Invention
The present invention relates to a positioning device and more particularly to an improved device for incrementally positioning at least one anode relative to the cathode of an electrolytic cell.
2. Description of the Prior Art
Electrolytic cells or pots, such as those used in the production of aluminum, basically consist of a carbon lined steel tank or vessel containing molten cryolite in which alumina is dissolved. Metallic aluminum is separated from the oxide (alumina) by passing an electric current through the solution at a temperature of approximately 970.degree. C. (1775.degree. F.). Carbon electrodes, or anodes, are suspended above each electrolytic pot. The depth of immersion of the anode in the electrolyte is adjustable and is carefully controlled. Usually, the anodes extend into the bath to within about two inches of the molten metal. Direct current is led into the electrolyte through the anodes and out through cathodic collector bars imbedded in the carbon lining.
In the reduction process, the electrolyte is maintained in a molten state and at a proper temperature by the heating action of the electric current passing through it from the anodes to the cathode. The anode-cathode distance should be frequently adjusted to maintain the desired bath temperature of about 960.degree.-980.degree. C. (1760.degree.-1796.degree. F.). Sufficient heat is lost at the surface of the electrolyte such that a frozen insulating crust, usually from 1 to 2 inches thick, forms at the top of the bath and around the anodes.
As the aluminum is electrolytically separated, oxygen is liberated at the carbon anode where it forms carbon dioxide while consuming the carbon anodes. As the carbon anodes are consumed, molten metal builds up in the bottom of the pot and the anodes must be adjusted periodically to assure a narrow spacing which maintains the voltage close to the decomposition voltage of the electrolyte. Proper frequent adjustment of the anode position will increase the efficiency of the electrolytic pot resulting in increasing production, reducing electricity requirements per pound of metal, reducing the rate of consumption of carbon anodes and reducing the possibility of short circuiting with associated problems.
To illustrate the present electrical inefficiency, a current of 1000 amp should deposit approximately 0.74 pound of aluminum per hour at 100% current efficiency. For various reasons, the best commercial current efficiency is about 90%. The theoretical voltage required for the decomposition of the alumina with a carbon anode is a little less than 2 volts; the actual voltage at the cell terminals is 4.5 to 7.0 volts. One way to reduce this inefficiency is to maintain a more accurate anode-cathode distance.
Electric motors such as those disclosed in U.S. Pat. No. 3,844,913 are presently used to position anodes. It has been found that the response of the present electric motors to a signal is highly variable, in part, because of the varying resistance to anode movement imparted by the frozen bath crust which has formed around the anode in the reduction pot. This crust can put a load in excess of 10,000 pounds on the anode and cause the motor to stall or become overloaded. To illustrate further, in a computerized reduction system, such as those disclosed in U.S. Pat. Nos. 3,627,666 and 3,900,373, each anode in the pot line may be consecutively surveyed to determine its proper position. The survey may determine that a specific anode should now be lowered 0.120 inch. If the electric motor takes 1.0 second to move the anode 0.040 inch, a 3.0 second timed pulse is sent to the electric motor. Under ideal conditions the anode would be lowered 0.120 inch and the computer would record this adjustment accordingly. However, in actual operation the crust may have imparted such a load on the anode that a 3.0 second timed pulse sent to the motor caused the anode to be lowered less than 0.120 inch, or possibly not be lowered at all. Therefore, although the proper position of the anode is accurately determined, and the correct time pulse is sent to the electric jack motor, there is no assurance that the anode has responded accordingly. The anode-cathode distance will be inaccurately adjusted, and this error will not be discovered until that specific anode is surveyed again, whereupon the anode may repeatedly be inaccurately positioned.
Also, there is an inherent safety hazard associated with the operation of three phase alternating current induction motors installed in a smelting pot which may be at a potential from zero to almost one thousand direct current volts relative to ground potential. This two voltage level condition creates a risk of injury or death to the operators.
Accordingly, an improved positioning device is required which will assure that an anode to be positioned in an electrolytic pot accurately responds to positioning commands to assure more precise control of the reduction process. Also, it would be beneficial if the employment of the improved positioning device would utilize nonelectrical-conducting lines to eliminate the present two levels of voltage in the reduction pot.